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Liaison International port infrastructures evolve slowly, and the effects of mobility are increasingly global. The answers to those fundamental questions, we found, depend largely on only a few factors. Historical data suggest that, throughout the world, personal income and traffic volume grow in tandem. As average income increases, the annual distance traveled per capita by car, bus, train or aircraft (termed motorized mobility, or traffic volume) rises by roughly the same proportion. The average North American earned $9,600 and traveled 12,000 kilometers (7,460 miles) in 1960; by 1990 both per capita income and traffic volume had approximately doubled. In developing countries the relation has been less tight. Between 1960 and 1990 the average income in China tripled, but motorized traffic volume rose 10-fold, to 630 kilometers. This discrepancy reflects, in part, the fact that growing wealth allows the poor to …